The present invention relates to a voltage supply circuit used as a power supply circuit of for example a liquid crystal display device (LCD panel).
For example, as a voltage for driving a LCD panel employing the thin film diode (referred to as MIM), three levels, that is, a signal electrode drive voltage VD, a high level selection voltage VH, and a low level selection voltage VL become necessary.
At this time, the signal electrode drive voltage VD, high level selection voltage VH, and the low level selection voltage VL must satisfy the following conditions when a virtual selection voltage is VSEL:
VL =xe2x88x92VSEL
VH =VD +VSELxe2x80x83xe2x80x83(1)
Conventionally, in order to obtain these signal electrode drive voltage VD, high level selection voltage VH, and low level selection voltage VL, a so-called charge pump type voltage generation circuit has been used.
In a voltage supply circuit generating the above three voltages by a voltage generation circuit of the charge pump type, however, there were problems in that equation (1) could not be satisfied with a sufficient precision (particularly when turning on the power) and in that the number of parts of the circuit was too large due to the wide variable range of the virtual selection voltage VSEL (generally about 2 V to 20 V).
The present invention was made in consideration with such a circumstance and has as an object thereof to provide a voltage supply circuit capable of starting up a system while holding symmetry of the high level selection voltage and the low level selection voltage and capable of decreasing the number of parts of the system without the need of a multistage charge pump circuit.
To attain the above object, a voltage supply circuit of the present invention comprises a first voltage generation circuit including a chopper type switching regulator for comparing a first reference voltage and an output feedback voltage and generating a first output voltage, a second voltage generation circuit including a chopper type switching regulator for generating a second output voltage, a third voltage generation circuit including a charge pump for generating a third output voltage in accordance with the second output voltage, a virtual reference voltage generation circuit for generating a virtual reference voltage corresponding to the first output voltage, an intermediate voltage generation circuit for generating an intermediate voltage between the second output voltage and the third output voltage, a comparator for comparing!the virtual reference voltage and the intermediate voltage and outputting an error signal in accordance with the comparison result, and a control circuit for receiving as input the error signal and controlling operations of the second voltage generation circuit and the third voltage generation circuit so that the virtual reference voltage and the intermediate voltage become equal.
Also, in the present invention, the control circuit makes the third voltage generation circuit operate when a voltage difference between the virtual reference voltage and the second output voltage is larger than the voltage difference between the virtual reference voltage and the third output voltage and makes the second voltage generation circuit operate when the voltage difference between the virtual reference voltage and the second output voltage is smaller than the voltage difference between the virtual reference voltage and the third output voltage.
Also, in the present invention, the virtual reference voltage is a middle point potential between the first output voltage and a ground potential, and the intermediate potential is the middle point potential between the second output voltage and the third output voltage.
Further, in the present invention, the second output voltage is higher than the third output voltage.